security/openvpn: bugfix update to 2.5.4

adds openvpn-examples(5) manual page

Changelog:
https://github.com/OpenVPN/openvpn/blob/release/2.5/Changes.rst#overview-of-changes-in-254
(cherry picked from commit cf4dd6bbfe80d93a23f2f7d140ed5dd764d13e96)